Process of charging heavy gas into a gas-filled cable



Sept. 24, 1968 J. J. WALKER GAS INTO A GASFILLED CABLE PROCESS OF CHARGING HEAVY Filed April 22, 1965 NN v 66644 .66;

INVENTOR.

JAMES J. WALKER United States Patent 3,403,063 PROCESS OF CHARGING HEAVY GAS INTO A GAS-FILLED CABLE James J. Walker, White Plains, N.Y., assignor to Anaconda Wire and Cable Company, a corporation of Delaware Filed Apr. 22, 1965, Ser. No. 449,936 9 Claims. (Cl. 156-48) My invention relates to a process of recharging gasfilled cable with high-dielectric-strength gasses and particularly to such a process that precludes the possibility of introducing any moisture into the cable.

Gas-filled cables are extensively used for the transmission of electric power at high voltage. These cables comprise a conductor, an insulation wall built up of oiled helical paper tape wrapping around the conductor, and a sheath such as a lead sheath surrounding the insulation. The cables are filled with nitrogen under substantial pressure, such as 55 p.s.i. absolute, to maintain high dielectric strength in the insulation and suppress ionization. It not infrequently happens that the sheaths of such cables deteriorate over a period of years so that it becomes desirable to decrease the pressure within the cable. This can be done safely if a heavy gas having high dielectric strength is substituted for the nitrogen, or a significant portion of the nitrogen, in the insulation. However, the practical process of making such a substitution has pre sented problems, some of which were entirely unexpected.

It was found, for one thing, that the mere introduction of a heavy gas of known ionization suppression value does not have the expected result of increasing the dielectric properties of the insulation. I now believe that this is due to the difliculty of introducing the heavy gas into the wall of oiled paper which is already saturated with nitrogen. Theoretically it might be considered that the nitrogen could first be exhausted from the cable but, in actual practice, this is not feasible since the application of vacuum to the cable for the purpose of withdrawing the nitrogen might draw in moist air through faults in the sheath. No moisture can, of course, be tolerated in high voltage cables of the type under consideration so that it becomes necessary to keep the cable under positive pressure at all times. This is particularly true for old cables of the type for which my invention has the greatest applicability.

I have discovered, however, that the above-mentioned obstacles can be overcome by my new process for introducing a heavy gas, such as sulfur hexafluoride, octafluorocyclobutane, hexafiuoroethane and perfluoro-butyl tetrahydrofuran, which are all electronegative gasses with high dielectric strength and high dew points, into a gasfilled cable with a gas-permeable wall of insulation and containing a relatively light pressurized gas such as nitrogen which has a low dew point. This process comprises the steps of reducing the gas pressure in the cable to a value slightly above atmospheric, holding the cable at this pressure until the pressure within the cable is substantially uniform both longitudinally and radially, and then introducing the heavy gas into the cable at one end so as to flush the original light gas from the longitudinal gas channels. Then, by interrupting gas flow from the cable, continue to introduce heavy gas until the pressure within the cable is substantially greater than atmospheric pressure, preferably at least doubled. I hold the cable at this high pressure until the pressure within the 3,403,063 Patented Sept. 24, 1968 ice cable is again substantially uniform, both longitudinally and radially, and then I repeat the foregoing steps until the heavy gas has saturated the wall of insulation. To recover the expensive heavy gas, I prefer to cool the gas mixture that is withdrawn from the cable in each step to separate the light and heavy gasses.

A more thorough understanding of my invention can be gained from the appended drawing. The drawing is a schematic of a cable and apparatus for applying my method.

In the drawing a gas-filled cable 10 has a conductor 11 surrounded by multiple layers of oiled paper-tape insulation 12 inclosed in a lead sheath 13 into the inner surface of which there have been formed a plurality of lengthwise gas channels 14. The sheath 13 is reinforced with a helical wrapping of steel tape 16 over which there is another sheath 17 of lead or the like or a jacket of rubber, polyethylene or the like. The cable has been terminated by two potheads 18, 19 of conventional design, which are inverted so that the oil in the potheads will not drain into the cable. Stems 21, 22 into the bells of the potheads 18, 19 provide means to admit and remove gas from the cable 10. The stem 22 is connected by a pipe 23 to a pump 24 which is connected through a valve 26 to a low-temperature, high-pressure condenser 27 cooled by refrigerated pipes 28. The condenser 27 is connected through a valve 29 and pipe 31 to a T 32 which is connected to the stem 21. The other opening of the T 32 is connected through a valve 33 and pipe 34 to a tank 36 of heavy gas. A pressure gage 37 is connected to the pipe 23. An exhaust vent 38 having a valve 39 is connected to the condenser 27.

When it is desired to substitute heavy gas for the nitrogen which saturates the insulation 12 and fills the channels 14 of the cable 10, which may have an initial pressure of 40 p.s.i.g., the valves 26 and 39 are opened to reduce the pressure very slowly until the gage 37 shows only a slight positive pressure. The valve 26 is then shut until the pressure within the cable has stabilized as indicated by a steady condition of the gage 37. This may take several days for a long cable, and if the pressure builds up during this time on the gage 37, it may again become necessary to open the valve 26 to let the gas out, never, however, permitting the pressure in the cable 10 to drop below atmospheric. After the cable has stabilized, heavy gas is introduced through the valve 33, the valve 29 of course, being closed, until a pressure is reached of 25 p.s.i.g. on the gage 37.

During the initial portion of this step I prefer to leave the valves 26 and 39 open until the heavy gas has flushed the nitrogen from channels 14 as determined by the appearance of heavy gas at the pipe 38, or by other known methods of determining the presence of heavy gas such as its condensation in a cold trap, or detection by a halogen detector in a known manner. With the valve 26 closed the cable is again permitted to come to pressure equilibrium, introducing more heavy gas, if necessary. In the event that the pressure in the tank 36 is not sufficient to raise the pressure in the pipe to the value that is desired, a pump may be introduced into the line of the pipe 34 in a known manner. With the valves 29 and 39 closed and the valve 26 open, the pump 24 is then operated to pump the gas from the cable 10 into the condenser 27 until the pressure shown by the valve 37 has been reduced to a point where it is once again only slightly above atmospheric. Refrigerant is circulated through the coils 28 to condense the heavy gas, the required temperature depending upon the pressure in the condenser and the dew point of the heavy gas being condensed. The pump is then stopped, the valve 26 is closed and the cable is again held until the pressure is stabilized within it. With regard to stabilization of the pressure within the cable it should be noted that after the pump has been shut off pressure begins to build up at the gage 37 from two sources, one is an additional fiow of gas longitudinally from the other end of the cable, and the other is a flow of gas radially outwardly through the wall of insulation. In a long cable an appreciable length of time is required for gas to flow through the channels 14 and in a high-voltage cable with a thick wall of insulation it may take hours for the pressure at the conductor to drop to the pressure in the channels. When the pressure is being increased the same mechanism applies in reverse. The length of time required for the cable to come to pressure equilibrium will depend, then, upon its length and upon the thickness of its insulation.

When the heavy gas has condensed in the condenser 27 the nitrogen is discarded by opening the valve 39. The valve 39 having again been closed, the condenser is warmed to vaporize the heavy gas and, the cable having come to pressure equilibrium, the valve 29 is opened to introduce the heavy gasses back into the cable. This is supplemented by gas from the tank 36 until the cabl is again brought up to the 25 p.s.i.g. pressure as indicated by the gage 37. The cycles of low and high pressure are repeated until the nitrogen in the insulation has been substantially replaced by heavy gas as determined by ionization initiation tests or by analysis of the mixture removed by the pump 24 from the cable.

Although I have so far limited my description to a single-conductor cable my discovery is not limited thereto and, in particular, is very useful for rejuvenating threeconductor cables wherein, as is well known, three insulated conductors are combined within a single sheath. Cables of this type do not form gas channels by fluting the sheath, but have longitudinal tubes extending lengthwise in the interstices formed by the conductors. In long cables at least one of these tubes is usually formed with an imperforate wall so as to transmit gas pressure rapidly from one end to the other of the cable. When such a threeconductor cable is rejuvenated by my method the light gas is flushed out of the tube or tubes by the heavy gas and then, before the pressure is built up of heavy gas, the tubes are valved off or otherwise interrupted.

I have discovered that once the heavy gas has saturated the cable insulation either by the methods hereinabove described or because it was used as the original dielectric, it does not diffuse readily therefrom and I have taken advantage of this discovery in the process of making cable splices. To prevent the access of moisture into a cable during splicing operations it has been the art-accepted practice to keep a flow of inert gas leaving the cable at the splice area. Such a practice is prohibitively expensive if, for cables with heavy gas dielectric, the heavy gas is continually wasted during a splicing operation, but I have found that other dry gas, such as nitrogen, can be flowed through the cable channels during the splicing operation without losing substantial quantities of the heavy gas entrapped in the insulation. When the splice is completed the light gas is flushed from the cable channels with heavy gas.

My invention has application to electronegative gasses that are substantially heavier than nitrogen since I have found that these do not diffuse readily into the nitrogen saturated wall of cable insulation. Particularly, my process can be used with advantage for the introduction into a cable of the fiuorocarbons such as sulfur hexafluoride,

trifluoromethyl sulfur pentafiuoride, octafluorocyclobutane, hexafluoroethane, perfluoro-butyl tetrahydrofuran and mixtures of such gasses. A mixture that has particular merit for the rejuvenation of medium high pressure cables, designed originally for operation at about 55 'p.s'.i. absolute will comprise 40 mole percent hexafluoroethane and 60 mole percent octafluorocyclobutane, saturated with perfiuoro-butyl tetrahydrofuran. Such a mixture will permit the cable to be operated at only 35 psi. absolute with dielectric strength equal or exceeding theoriginal design.

Other heavy gasses that can be used within the scope of my invention include perfiuoro-n-butane, dichlorodifiuoromethane, monochlorotrifluoro methane, dichlorotetrafluoro ethane, monochlorodifiuoromethane, perfiuoropropane, monochloropentafluoroethane, monobromomonochlorodifiuoromethane and dichlorotetrafluoroethane.

When less expensive electronegative gasses are being used, particularly those with low dew points such as would require a high degree of refrigeration in the condenser 27, it might not be economical to go through the liquifaction recovery step of my process but cheaper to discard the mixed gasses pumped from the cable. This will depend upon the length of cable being processed, as well as the type of gas. The recovery of octafluorocyclobutane which is expensive and has a high dew point is almost always justified while the recovery of sulfur hexafluoride is usually impractical.

I have invented a new and useful process for rejuvenating high-voltage electric cables for which I desire an award of Letters Patent.

I claim:

1. The process of introducing a heavy gas into a gasfilled cable having a gas permeable wall of insulation and containing relatively light gas under pressure, comprising the steps of:

(A) reducing the gas pressure in said cable to a value slightly above atmospheric pressure,

(B) holding said cable slightly above atmospheric pressure until the pressure within said cable is substantially uniform, both longitudinally and radially,

(C) introducing heavy gas into said cable until the pressure within said cable is substantially greater than atmospheric pressure,

(D) holding said cable at substantially greater than atmospheric pressure until the pressure within said cable is substantially uniform, both longitudinally and radially, and

(E) repeating the foregoing steps until said heavy gas has saturated said wall of insulation.

2. The process of claim 1 wherein said light gas is nitrogen,

3. The process of claim 1 wherein said heavy gas is a fluorocarbon having high dielectric strength.

4. The process of claim 1 wherein said heavy gas is selected from the group consisting of sulfur hexafluoride, octafluorocyclobutane, hexafluoroethane, perfluoro-butyl tetrahydrofuran, and trifluoromethyl sulfur pentafluoride.

5. The process of claim 1 wherein said heavy gas is introduced until the pressure has at least doubled.

6. The process of introducing a heavy gas having a rela-' tively high dew point into a gas-filled cable having a gas permeable wall of insulation and containing relatively light, low dew-point gas under pressure comprising the steps of: r

(A) reducing the gas pressure in said cable to a value slightly above atmospheric pressure,

(B)' holding said cable slightly above atmospheric pressure until the pressure within said cable is substantially uniform, both longitudinally and radially,

(C) introducing said heavy gas into said cable until the pressure within said cable is substantially greater than atmospheric pressure,

(D) holding said cable at substantially greater than atmospheric pressure until the pressure within said cable is substantially uniform, both longitudinally an radially,

(E) withdrawing the mixture of gasses within said cable until the pressure therein is slightly above atmos- 9. The process of claim 6 wherein said heavy gas is pheri introduced until the pressure has at least doubled. (F) cooling said gas mixture to separate the light and heavy gasses and References Clted (G) repeating the foregoing steps until said heavy gas 5 UNITED STATES PATENTS has saturated said wall of insulation. 2 2 1 7 11/1940 Cooper 7. The process of claim 6 within said light gas is 2,432,568 12/1947 Gambitta 17425 X nitrogen. 2,853,540 9/1958 Camilli et al. 174-16 X 8. The process of claim 6 wherein said heavy gas is 5 5/1959 Wolfe 17416 X selected from the group consisting of sulfur hexafluoride, v octafluorocyclobutane, hexafluoroethane, perfluoro-butyl EARL Pnmary .Exammer tetrahydro'furan, and trifiuoromethyl sulfur pentafiuoride. SAVOIE Ass'stam Examme 

1. THE PROCESS OF INTRODUCING A HEAVY GAS INTO A GASFILLED CABLE HAVING A GAS PERMEABLE WALL OF INSULATION AND CONTAINING RELATIVELY LIGHT GAS UNDER PRESSURE, COMPRISING THE STEPS OF: (A) REDUCING THE GAS PRESSURE IN SAID CABLE TO A VALUE SLIGHTLY ABOVE ATMOSPHERIC PRESSURE, (B) HOLDING SAID CABLE SLIGHTLY ABOVE ATMOSPHERIC PRESSURE UNTIL THE PRESSURE WITHIN SAID CABLE IS SUBSTANTIALLY UNIFORM, BOTH LONGITUDINALLY AND RADIALLY, (C) INTRODUCING HEAVY GAS INTO SAID CABLE UNTIL THE PRESSURE WITHIN SAID CABLE IS SUBSTANTIALLY GREATER THAN ATMOSPHERIC PRESSURE, 